Pipelines carrying a variety of gaseous and volatile liquid products are extensively employed in today's industry as a safe and efficient means of transportation. New pipelines must be placed into service initially (i.e., commissioned), and older lines occasionally are taken out of service in order to perform some desired maintenance and are then recommissioned. Among the reasons for taking a pipeline out of service (i.e., decommissioning) are: hydrostatic testing to recertify or upgrade the pipeline's ability to be used at higher operating pressures; performance of construction work on the pipeline; a change in the product transported by the pipeline. Pipelines, or sections thereof, may need to be relocated because of highway work, the necessity to deepen a canal, or because of increases in the population surrounding the pipeline. It may also be necessary to replace valves, fittings or a damaged section of the pipeline or add a new connection to service a customer or supplier.
Typically, decommissioning and recommissioning a petrochemical pipeline will include the steps of decommissioning by removing the product from the pipeline, and flaring any residual product remaining therein. Any necessary construction, upgrading, or cleaning of the pipeline can then be performed. Usually, the pipeline is then filled with water for hydrostatic testing. After pressure testing, the water is removed, the pipeline is cleaned and dried to a specific low dewpoint (to avoid the problems of water contamination of the product) and the pipeline is inerted with nitrogen for recommissioning. U.S. Pat. No. 3,864,102 describes a process and system for drying pipelines to specified dewpoints using dry air.
In order to recommission the pipeline, the nitrogen must be displaced by the desired petrochemical. Before returning to service, product purity must be established and the line safely filled to operating pressure. The terms "commissioning" and "recommissioning" are used interchangeably herein to refer to a process whereby a first inerting gas, normally nitrogen, in a pipeline is replaced with the desired product at the desired purity and pressure.
Safety and economics are two primary concerns for any proposed pipeline operation. Thus, a commissioning process which brings the purity and pressure of potentially explosive products such as ethylene or propylene up to specification quickly, but at the risk of damage to the pipeline or reduced safety to operating personnel, is not acceptable. Similarly, a process which uses large quantities of product to push nitrogen from a line results in wasted product/nitrogen mixtures that must be flared or otherwise disposed of would also be unacceptable. Therefore, the measure of whether a commissioning process actually constitutes a viable improvement over known methods requires consideration of its safety and economic benefits (considering both the cost of wasted product and the cost attributed to the time the line must remain out of service) as well as its ability to bring the line back into service with product at desired pressures and purities.
The main problems posed by recommissioning are (1) the fast and economical purging of nitrogen so as to obtain uncontaminated products in the line, and (2) possible damage to the pipeline due to cold temperatures to which it may be subjected during the process. The latter can be a problem when the pipeline is to be recommissioned with a product at a pressure significantly higher than that of the nitrogen inerted line. The pressure drop of a petrochemical product entering a pipeline can cause rapid drops in temperature and, consequently, potential damage to the carbon steel pipeline.
Due to the demands of the expanding petrochemical industry, and the aging of the present pipeline system, the need for decommissioning, repairing, cleaning, drying and recommissioning pipelines is increasing. Given the value of today's petrochemical products, a recommissioning procedure, which brings the product purity to acceptable levels as quickly as possible, is desirable. Furthermore, processes and equipment which reduce the possibility for damage to the pipeline due to cold temperature conditions which can occur during the recommissioning process are also highly desirable.